It is a common mistake for people to assume that switching on a firewall makes them safe. This is not the case and, in fact, has never been the case. Each system has distinct security needs, and taking the time to customize its security layout gives you maximum security and the best performance. The following list summarizes the most common mistakes:
After you have ruled out these mistakes, you are onto the real problem: Which attack vectors are open on your server? In Internet terms, this comes down to which services are Internet-facing and which ports they are running on. Two tools are often used to determine your vulnerabilities: Nmap and Nessus. Nessus scans your machine, queries the services running, checks their version numbers against its list of vulnerabilities, and reports problems. Although Nessus sounds clever, it does not work well in many modern distributions (Fedora included) because of the way patches are made available to software. For example, if you're running Apache 2.0.52 and a bug is found that's fixed in 2.0.53, Fedora back-ports that patch to 2.0.52. This is done because the new release probably also includes new features that might break your code, so the Red Hat team takes only what is necessary and copies it into your version. As a result, Nessus will see the version 2.0.52 and think it is vulnerable to a bug that has in fact been backported. The better solution is to use Nmap, which scans your machine and reports on any open TCP/IP ports it finds. Any service you have installed that responds to Nmap's query is pointed out, which enables you to ensure that you have locked everything down as much as possible. To install Nmap, select System Settings, Add/Remove Applications. Near the bottom of the list of applications is System Tools, where you can select Nmap-front-end and Nmap. Although you can use Nmap from a command line, it is easier to use with the front endat least until you become proficient. To run the front end, select Actions, Run Application and run nmapfe. If you want to enable all Nmap's options, you need to switch to root and run nmapfe from the console. The best way to run Nmap is to use the SYN Stealth scan, with OS Detection and Version Probe turned on. You need to be root to enable the first two options (they are on by default when you are root), but it is well worth it. When you run Nmap (click the Scan button), it tests every port on your machine and checks whether it responds. If it does respond, Nmap queries it for version information and then prints its results onscreen. The output lists the port numbers, service name (what usually occupies that port), and version number for every open port on your system. Hopefully, the information Nmap shows you will not be a surprise. If there is something open that you do not recognize, a hacker might have placed a backdoor on your system to allow her easy access. You should use the output from Nmap to help you find and eliminate unwanted services. The fewer services that are open to the outside world, the more secure you are. |